The present invention relates to polymer-based electroluminescent devices, and more particularly, to a class of precursors for generating the electrically conducting polymers that provide electroluminescent layer in such devices.
Polymer-based electroluminescent devices (PLEDs) have the potential for providing inexpensive alternatives to alpha-numeric displays and x-y addressable displays. PLEDs also have the potential to provide an alternative to back lighted, liquid crystal displays. A simple PLED may be constructed from an electroluminescent layer sandwiched between an electron injection electrode and a hole injection electrode. The electroluminescent layer is typically constructed by depositing a conjugated or conductive polymer on one of the electrodes. Devices based on poly(p-phenylenevinylene) (PPV), or derivatives thereof, have been demonstrated with sufficient quantum yields to be commercially attractive. More complicated devices utilize electron and hole transport layers between the above mentioned electrodes and the electroluminescent layer. The electroluminescent layer generates light when holes and electrons recombine in the layer.
The deposition and patterning of the electroluminescent layer present significant technical problems that must be overcome before economically attractive devices can be fabricated. If the conjugated polymer is soluble in a solvent, a thin film can be made by the spin-coating of a polymer solution. While spin-coated polymer films having good electro-optical properties can be obtained in this manner, the adhesion of spin-coated film to the underlying layer is often insufficient. In addition, many attractive polymers are not sufficiently soluble to be applied via spin-coating.
Spin-coating and other processes in which the entire substrate is coated, present additional problems in multi-color displays in which different xe2x80x9cpixelsxe2x80x9d must be coated with different polymers. The deposition of each layer requires a three-step procedure consisting of a masking step to protect areas that are not to be coated, the spin-coating step, and a mask removal step. In addition to the increased complexity of the masking steps, the solvents utilized with conventional masking systems are often incompatible with the polymers being deposited. Accordingly, it would be advantageous to provide a system that does not require such masking operations.
To avoid these limitations, thin films of various conjugated polymers have been prepared by the electrochemical polymerization of a monomer solution. In principle, this method can also provide a film of conjugated polymer that is not soluble in any solvent, and hence cannot be deposited by spin-coating. In this method, a monomer of the polymer is dissolved in a solvent and then the polymer film is deposited on an electrode by maintaining the electrode at a potential with respect to the solvent. The solvent is chosen such that the monomer is soluble therein, while the formed conjugated polymer is insoluble. Unfortunately, the quality of such polymer films leaves much to be desired. In many cases, the film consists of a layer of powdery material that is precipitated in the vicinity of the electrode rather than being deposited onto the electrode. This precipitation problem results from the poor solubility of polymer units having a few monomers connected together that are formed in the vicinity of the electrode. Attempts to overcome this problem by utilizing parent monomers with flexible alkyl groups to increase the solubility of short chain polymer units have not yielded films of a quality needed for electronic devices. In addition, this approach also has a disadvantage of reducing the concentration of electroactive functional groups in the film structure, and hence, the efficiency of light generation by the films.
Broadly, it is the object of the present invention to provide an improved precursor material for use in the generation of electrically conducting polymers such as those used in PLEDs.
It is another object of the present invention to provide a precursor material that can be utilized to generate an electrically conducting polymer layer via electrochemical polymerization.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is a precursor polymer that can be electrochemically polymerized to form an electrically conducting polymer. A precursor according to the present invention includes a plurality of electrochemical polymerizable monomers, each monomer having first and second polymer-forming active sites that can be joined by electrochemical polymerization. The monomers also include third and fourth polymer-forming active sites that can be joined chemically in solution. The monomers in the precursor polymer are linked by the third and fourth polymer-forming active sites. The monomers are chosen such the precursor polymer is soluble in a predetermined solvent while the polymer resulting from the electrochemical polymerization of the precursor polymers via the first and second polymer-forming sites is electrically conducting and insoluble in the solvent. The monomers can be synthesized from fluorene, triophene, pyrrol, biphenyl, poly(vinyl carbazole) or poly (vinyl oxy thiophene). Monomers comprising dimers chosen from this group can also be utilized. The monomers may also include a spacer group bonded to one of the first or second polymer-forming active sites. Spacer groups consisting of (CH2)n, (OCH2)n, or (OCH2CH2)n, where 1xe2x89xa6nxe2x89xa620 may be utilized for this purpose.